The present invention relates to the field of electronic impedance matching devices, or electrical energy transfer devices. More particularly, the present invention relates to transformer devices utilizing transmission line principles to achieve very wide bandwidths.
In the electronic engineering fields of communications (e.g., mobile radio, cable television), power amplification, antenna matching, microwave circuits, and digital systems, means are frequently required for processing electrical energy spread across a wide band of frequencies without wasting any significant amount of the energy in any frequency range. However, early step-up and step-down transformers for matching impedances of substantially different magnitudes exhibited resonances of stray capacitances and inductances in the apparatus which substantially limited the high-frequency performance.
Later, C. L. Ruthroff in his U.S. Pat. No. 3,037,175 "Broadband Transformers" issued May 29, 1962, (hereby incorporated by reference into the present specification) disclosed an apparatus said to have the broadband characteristics of a parallel wire transmission line and the impedance transformation characteristics of a centertapped autotransformer. Construction and testing of such apparatus is described in an article "Designing Toroidal Transformers to Optimize Wideband Performance" by H. L. Krauss et al., Electronics, Aug. b 16, 1973, pages 113-116. The latter authors warn that care in construction is necessary so as to avoid uneven wire spacing that is said to introduce impedance variations that degrade high-frequency performance. Both in theory and in practice such a transformer apparently has limits to its low- and high-frequency performance even when perfectly constructed. Accordingly, it is of interest to the art to find an advantageous alternative transformer apparatus for wideband operation in which the limitations in the prior art can be relaxed or obviated.